As discovered by Stephen L. Trokel ("Excimer Laser Surgery of the Cornea", American Journal of Ophthalmology, December 1983), far ultraviolet radiation from an excimer laser can be used to change the refractive power of the cornea of an eye. The radiation ablates away corneal tissue in a photo decomposition that does not cause thermal damage to adjacent tissue, and can be called photorefractive keratectomy. A similar photo decomposition of corneal tissue can achieved with an infrared laser operating near 2.9 micrometers, where thermal damage to adjacent tissue is minimized by the high absorption of water.
L'Esperance U.S. Pat. No. 4,665,913 describes procedures for changing the contour of the anterior surface of the cornea of the eye by directing pulses from an excimer laser in a scanning pattern that moves around the cornea. The laser pulses first ablate and remove the epithelium of the cornea, and then the ablation penetrates into the stroma of the cornea to change its contour for various purposes, such as correcting myopia or hyperopia. Schneider et al. U.S. Pat. No. 4,648,400 suggests radial keratectomy with an excimer laser that also ablates away the epithelium before penetrating into, and changing the contour of, the stroma of the cornea.
I have discovered inherent inaccuracies in the process of ablating corneal tissue with an excimer laser beam, and I have found ways of substantially improving the accuracy of the process. Understanding my invention, though, requires an understanding of the excimer laser beam and the structure of the cornea.